The scene is the culmination of the movie-long reveal of the origins of the killer creatures that cause so much havoc in "Alien" and its sequels.
In "Prometheus," the creature is the result of a somewhat complicated process:
- A human (Holloway) was infected by the black goo
- The infected human created a new organism within a non-infected human (Shaw)
- This new organism grew to be an octopus-like mutant
- The octopus-like mutant created another new organism within an Engineer
The second new organism's "birth" consisted of it springing out of its host's rib cage, just like the creatures in "Alien" and its sequels -- which it also resembles physically. (Notably, the alien at the end of Prometheus looks somewhat different from the aliens in the other Alien movies. I attribute this to the fact that in Prometheus, the alien is spawned from an Engineer, while in the other movies the aliens are spawned from humans.)
One of movie's ironies is the first action on the list above was taken by an android, who was a creation of man, who in turn was a creation of the Engineers.